In a wireless local area network (WLAN) operating in infrastructure mode, a STA typically performs scanning in order to estimate which AP is the best candidate to serve it. Scanning performed by the STA can be passive or active. In passive scanning, the STA listens for beacon frames sent by the APs. In active scanning, the STA sends out probe requests and the APs respond by sending a probe response to the STA.
In order to enhance coverage and increase throughput, APs can be equipped with advanced antenna structures, which allow them to change the radiation pattern (the beam) they use. This is a function of the position of the STA to which the AP sends packets and/or of the time-varying channel. Because the beacon frames are not aimed at a particular STA, but rather at a multitude of STAs, they tend to be sent uniformly across all directions (i.e., with an omni-directional beam). Similarly, because it is desirable for an AP to hear the packets from all its associated STAs, the AP typically listens to the channel using an omni-directional beam. This type of beam does not necessarily allow the AP to determine which beam is the best to serve a STA, even after it has received a probe request from the STA. Therefore, a probe response also tends to be transmitted using an omni-directional beam.
A STA uses the beacons (passive scanning) and the probe requests (active scanning) to estimate, among other things, the quality of the radio links (e.g., signal to noise ratio (SNR)) that it could obtain from the different APs. Beacons and probe responses are typically sent by the APs using an omni-directional beam for the reasons described above. This creates a situation where a STA may estimate, based on the received signal quality it perceives from the beacons and/or probe responses, that a given AP is the best candidate, when in fact this AP will perform worse than another AP for the transmission of traffic frames which can be sent with directional beams.
FIG. 1 shows an exemplary WLAN 100 including a STA 102 and two APs, AP_A 110 and AP_B 120. AP_A 110 can utilize an omni-directional beam 112 and a plurality of directional beams 114, 116, 118. AP_B 120 can utilize an omni-directional beam 122 and a plurality of directional beams 124, 126, 128.
The STA 102 measures the received power of the various beams as shown in Table 1.
TABLE 1Measured Received Signal Power at STA 102Received power atSignal fromOn beam numberBeam typeSTA 102AP_A112omni−75 dBmAP_A116directional−85 dBmAP_A118directional−85 dBmAP_B122omni−80 dBmAP_B124directional−70 dBm
The STA 102 can receive stronger beacons and/or probe responses via omni-directional beam 112 from AP_A 110 than via omni-directional beam 122 from AP_B 120. However, AP_B 120 would be a better candidate than AP_A 110 for transmitting traffic frames, since AP_B 120 is in a better position to make use of its advanced antenna structure and focus the energy towards the STA 102 (via directional beam 124).